+1 Ace. I think there will be a 'mild' version with stock head and FI system and a 'wild' version with more. As to the REV limit, would it be possible to go past that with a different crank or is the head the limiting factor?
Scott
Scott,
That's a good question.
Piston speed is normally the determining factor for settting rev-limits, and that is determined by the stroke length of the crank, primarily. With a 90mm stroke, I determined that I would limit the Fireball mod to 6000rpm, even with a modded crank and a Carrillo rod and lightweight piston. I did this for reliability purposes for street use. Technically, you 'could' push that 90mm stroke alot faster, as long as you are wiling to take the risks of failure, and reduced lifetime of parts. This is commonly done in racing engines where winning is more important than longevity.
So yes, it is possible to push it higher from a certain point of view, but it gets progressively riskier and shorter-lifetime as you go up the scale.
6000rpm on a 90mm stroke is already well above the Pomeroy Dictum recommended levels for reliable piston speeds in a street engine which expects good longevity.
Bottom end strength can play a part in improving the chances of going up higher, but I seriously doubt that Enfield is putting stuff any better than what is in the Fireball into their stock engines.
But for really pushing the racing lmits, it could be pushed over 7000rpm, or maybe even to 8000rpm, for a short-term life-expectancy.
To give you a basic reference, a 90mm stroke Bulllet at 7000rpm would average 4130 feet/minute piston speed.
A Yamaha R6 with a 44.5mm stroke reaches approximately the same average piston speed at just over 14,100rpm.
So, for all intents and purposes, pushing an Enfield to 7000rpm presents the similar piston speed issues to the engine as an R6 doing over 14000rpm.
And we don't have the same sophistication of design in our engines as the R6 in terms of balance. Plus, our unbalanced system has much longer stroke, so our PEAK piston speeds would be even higher than the R6 would have as a peak piston speed.
And we see that even the Yamaha reaches its rev limit just above that 14krpm level, at 14.5krpm. So, they are limiting it there BECAUSE piston speeds cannot go higher in even that sophisticated engine with all it's hi-tech, and still expect any kind of longevity in the system.
We don't intend to push that far, so I think that 6000rpm or maybe a bit higher, is all I want to be doing in a street Enfield and expect any decent lifetime.
From the POV of valve-train stability, the hydraulic lifters begin to show up potential issues before the 6000 rpm limits that perhaps the bottom end of the engine could withstand. Lifter "pump-up" is a well-known issue in pushrod engines, and is generally expected when rpms get over 5500 rpm - 6000rpm in engines with hydraulic lifters. This is where Chinoy had his problems. There are ways to get around that problem, which typically amounts to essentially using a solid lifter when hi-rev applications are anticipated, and the roller part of the lifter can still be retained with a solid lifter. And there are some high-performance hydraulic lifters available too, but I don't know if any of them are suitable to go in this Enfield.
The next thing is that we see only a ~600rpm gap between existing rev limits and the 6000rpm limit that I felt was the outer limits for a street bike with this stroke length.
So then, we need to determine if we feel that extra 600rpm is worth doing all this tuff to get there, or if it's just better to work under the existing rev limits.
Since rpms are part of the hp equation, we need revs to get a higher hp figure.
Exactly how many rpms is up to the builder and user. Many things can factor into this decision.
However, torque is on the other end of the hp equation, and so increasing torque will also give us more hp, and we don't have to push the rpms up as high to get that, and so it is where most long-stroke tuners spend their efforts.
The Fireball is primarily a "torque build" with some added rev capacity to pull the hp number up a bit higher, and have enough revs to hit "the Ton" with our typical gearing.
A whole lot to making these decisions.
Many ways to build an engine.
Many types of budget considerations to be thought about.
So, it needs to get focused.
I made my focus with the Fireball, based on many factors, and it seems to have hit the target right in the bulls-eye. That is what we want to get out of any other build. A bulls-eye for what the desired end results are.
As for the head parameters, we only look at those AFTER we have set the intended rev-range requirement, and then we port and cam to suit.
The head is not really a "limiting factor" unless it's already too big in the ports, or it's not big enough in the ports, and how much the casting wil allow us to do with it.
Of course, there are some basic issues like port angles and stuff, but we are pretty much limited to what's there, unless we're making a whole new head, and even then we still have to fit the thing under the tank, and it's going to be priimarily side-draft no matter what happens.. We can make minor port angle changes perhaps, and do alot with size and shape of the port if the casting allows it, and we can do things with the chamber to reduce valve shrouding and improve flow into the chamber, and alot of things like that, though.
A pertinent example of that would be the Fireball, where we didn't increase the port size at all, and still picked-up 55 cfm in flow at a very moderate valve lift height that wouldn't stress the valve-train system much more than stock. So we got a huge flow increase of about 35%, improved combustion efficiency by using advanced atomization improvement techniques, and gained alot of power potential on the same basic port size as a stock Bullet. and to go along with it, we redesigned a large portion of the valve-train using high-performance and high-reliability parts, so that it would have no trouble at all with the job we were asking it to do.